Steel products for large structures, by virtue of advance in steelmaking techniques in recent years, have been improved in various respects, offering excellent matrix properties. In general, however, it is difficult to maintain, for example, the properties of weld heat affected zones, which appear in joints formed by welding, on a level comparable to the matrix properties provided in the course of production of a steel product. This is because welding heat coarsens the particles in a heat affected zone (hereinafter referred to as "HAZ"), rendering the steel brittle. That is, it is known that the size of grains in a HAZ greatly influences low-temperature toughness of the steel. This led to the development of a technique for the refinement of HAZ structures which is now in practical use.
For example, Japanese Unexamined Patent Publication (Kokai) No. 61-79745 proposes a technique where Ti is added to steel to disperse therein an austenite grain fine Ti oxides which are utilized as transformation nucli to develop transgranular ferrite, thereby refining the HAZ structure. Japanese Unexamined Patent Publication (Kokai) No. 61-238940 discloses that uniform dispersion of fine Ti oxides as secondary deoxidation products results in improved toughness in a HAZ.
Japanese Unexamined Patent Publication (Kokai) No. 01-228643 discloses that the addition of deoxidizers, such as Zr, Ti, Ce, Y, and Hf, to molten steel having regulated dissolved oxygen content followed by fine dispersion of MnS utilizing the resultant deoxidation products as nucli refine the steel structure, contributing to improvement in toughness in a HAZ and other steel properties. Japanese Unexamined Patent Publication (Kokai) No. 03-047664 teaches that MnO--SiO.sub.2 (Mn silicate) oxides are suitable as nucli for the precipitation of MnS.
Also in the case of steel sheets for cold working, it is known that fine dispersion and precipitation of MnS accelerate the precipitation of cementite in the step of continuous annealing, improving the workability.
AS described above, it is known that deoxidation products produced in steel and MnS can be utilized to improve the quality of the steel and that Mn silicate oxides are suitable as nucli for the precipitation of MnS. One means for improving the quality of the steel is to finely disperse many MnS precipitates. In order to precipitate a large amount of MnS, it is common practice to increase the content of S. Since, however, S in many cases adversely affects various properties of the steel, the S content is preferably minimized, to up to 0.01%.
For this reason, finding of optimal conditions of oxides for the precipitation of Fins becomes necessary for increasing the number of precipitated MnS while maintaining a low S content.
An object of the present invention is to provide steel slabs and steel products with a large amount of MnS being finely precipitated therein while maintaining low S content.
Another object of the present invention is to provide cast slabs containing oxides which permit such Fins to precipitate.
A further object of the present invention is to provide steel products having excellent properties in weld heat-affected zones.